The present invention relates to a process for the purification of vinylidene chloride and more particularly to a process for obtaining vinylidene chloride which no longer contains significant amounts of dichloroacetylene.
Dichloroacetylene is an impurity which is frequently encountered in vinylidene chloride and which has proved to interfere greatly with the polymerization of the latter. In effect, dichloroacetylene causes chain transfer reactions during the polymerization of vinylidene chloride, resulting in a very marked reduction in the degree of polymerization.
On the other hand, a very important outlet for vinylidene chloride polymers is the manufacture of transparent films for packaging. Hence, it is essential that the starting monomer should be colorless. Now it has been found the vinylidene chloride contaminated with dichloroacetylene discolors during storage, especially in mild steel containers, which makes the vinylidene chloride unsuitable for the manufacture of transparent polymers.
In order no longer to be objectionable, the dichloroacetylene content must not exceed about 70 ppm and preferably must be less than 50 ppm.
The presence of dichloroacetylene in vinylidene chloride is due to the process of manufacture of the latter. If vinylidene chloride is prepared by dehydrochlorination of 1,1,2-trichloroethane (or possibly 1,1,1-trichloroethane) containing small amounts of tetrachloroethanes and/or of trichloroethylene, it is found that the vinylidene chloride obtained contains dichloroacetylene arising from the dehydrochlorination of the tetrachloroethanes and/or of the trichloroethylene.
The usual techniques of purification of vinylidene chloride by distillation do not permit easy removal of the dichloroacetylene because the latter has a boiling point close to that of vinylidene chloride.